A person that is the prime example of a douchebag. Maybe the term douchebag is too light a word to describe them. They think that they're all that and a bag of chips, and refuse to accept that they suck at life. Oftentimes they call other people douches because they are in denial of their own douche-ness.
"I pop muh collah and get all da bitches followin me around."
Bystanders: "Man, that guy's not just a douche, he's a doucheous maximus!"
A person who is an extra large douche, especially one that makes an ass of himself as part of his general douchy-ness.
Derived from the Latin for hindquarters "Gluteus Maximus," where "Maximus" means "really big."
#1: "So, guess what Douchius Maximus said today?"
#2: "Tell me!"
#1: "He asked again if he could have the vacuum cleaner."
#2: "No! What did you say?"
#1: "I told him no."
#2: "Good."
#1: "Seriously. He has one, it's just not as nice. You don't have an affair with my friend, move out, leave me with three children, and then ask for the new vacuum cleaner because you took the old one. Kids are messy. You are not. You have a job. Go buy a fucking vacuum cleaner."